Abstract: Ultrasonic propagation differential method based on critical wave Lcr acoustic-elastic effect in the condition of uniaxial stress state was proposed. It raised the spatial resolution of Lcr wave stress measurement to a higher level. It was applicable in the evaluation of welding residual stress,which was a gradient stress field. Residual stress distribution in an undercarriage of high speed train with welding deformation was surveyed by this method in manufacturing shop. Residual stress in both quality and sub-quality areas were obtained and compared. The source of the contraction distortion was confirmed,and it provided reference to the working out of the shape-righting scheme.